1. Field of the Invention
The present invention relates to an electrophotographic method, and, more specifically, to an electrophotographic method in which an image consisting of powder is formed on an electrically insulating or electroconductive support.
2. Description of the Prior Art
Typical electrophotographic methods thus far developed, as is generally known, employ basically an photoconductive layer on an electro-conductive support. The photoconductive layer is charged in a uniform manner using a corona charger or the equivalent to a corona charger. As a result of the charging in a manner such as set forth above, the photoconductive layer is rendered appropriately photosensitive. Subsequently, image-wise exposure of the photoconductive layer is achieved using radiation of a spectral range which activates the photoconductive layer. Pairs of electrons and positive holes are generated in the areas of the photoconductive layer of an electrophotograph thus exposed to light, and these pairs of electrons and holes flow through the photoconductive layer, to thus dissipate the electrical charge present on the surface of the photoconductive layer.
In the manner as set forth above, an exposed area free from an electric charge, i.e., an area in such a state that a portion of an image is uncharged, in other words, an electrostatic latent image, is appropriately formed on the exposed section in such a manner as set forth above. This electrostatic latent image is converted into the final image using processes such as development, transfer, and fixation.
In this type of electrophotography, the electroconductive support in contact with the photoconductive layer plays a quite important role. To put it otherwise, the electroconductive support acts as either a grounding plate or the standard electrode at the time of corona charging. In addition, the electroconductive support functions as an electrode required at the time of dissipation of the electric charge of the exposed section. The latter function is especially important. When the photoconductive layer is subjected to corona charging or charging using some other means equivalent thereto, the electroconductive support has an electric charge of a reverse polarity to that induced on charging the photoconductive layer.
That is to say, an electric field is generated between the surface of the photoconductive layer and the electroconductive support due to the charging, and the photoconductive layer is thus rendered photoconductive. When the photoconductive layer in such a state is irradiated with light in the range of the spectrum which activates the photoconductive layer, pairs of electrons and positive holes are generated, and the electrical resistance of the photoconductive layer is reduced. As a result thereof, it is impossible for an electrical charge to be retained on the surface of the photoconductive layer, and the electric charge is dissipated.
To effect this dissipation of the electric charge, it is necessary for and electric current to flow through the photoconductive layer. To cause an electric current to flow through the photoconductive layer, an electric field is required, and for this purpose, an electroconductive support is required. The electroconductive support thus acts as an electrode.
As used herein, the term electroconductive support means a support such as a metal plate, a paper or another insulating support coated with a thin metal film or another electroconductive substance, a paper or another electroconductive substance under an appropriate relative humidity, which has a surface resistance of approximately 10.sup.10 .OMEGA.sq.
As described above, this electroconductive support is both important and indispensable for these prior electrophotographic methods. Therefore, it is quite difficult, generally, to form an image in an electrophotographic manner without an electroconductive support.
Furthermore, some methods analogous to the present invention have been described in British Pat. Nos. 990,438, and 1,198,497, and so forth. Each and every one of these methods involve a method of forming a powder image of on a support, especially on a material to be processed, by the employment of a photoconductive powder. The method disclosed in the specification of British Pat. No. 990,538 is a method of forming an image of a photoconductive powder on the surface of a processed material having the property of being more or less electroconductive. In addition, the method described in the specification of British Pat. No. 1,198,497 is a method having features in which a processed material whose surface is electrically insulating is appropriately surface treated, to thus render the surface (more or less) electroconductive.
As exemplified above, each and all of these methods merely proves to be effective only where the surface of a processed material is electroconductive. However, in the method of the present invention, a photoconductive powder image can be formed, in an appropriate and advantageous manner, even on the surface of a support which is either electrically insulating or electroconductive. In this respect, the method of the present invention is entirely different from any of the conventional methods, and is believed to be quite an important advance in the art.